1. Technical Field of the Invention
The present invention generally relates to data communication systems using radio equipment and, in particular, to a method and system for optimized data medium access for an ad-hoc radio channel.
2. Description of Background and Related Art
In the last decades, progress in radio and Very Large Scale Integration (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radiotelephones, can now be produced having acceptable cost, size and power consumption.
Although wireless technology is today focused mainly on voice communications (e.g., with respect to handheld radiotelephones), this field will likely expand in the near future to provide greater information flow to and from other types of nomadic devices and fixed devices. More specifically, it is likely that further advances in technology will provide very inexpensive radio equipment, which can easily be integrated into many devices. This will reduce the number of cables currently used. For instance, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals. Recently, a new air interface named Bluetooth™ was introduced to replace all cables between mobile phones, laptop computers, headsets, Personal Digital Assistants (PDAs), and so on. An introduction to the Bluetooth™ system can be found in “Bluetooth™—The universal radio interface for ad hoc, wireless connectivity,” by J. C. Haartsen, Ericsson Review No. 3, 1998.
The aforementioned radio communications are based on peer communications and ad-hoc networking. This means that the system is not based on a hierarchical scheme with a fixed infrastructure of base stations and portable terminals that communicate with the base stations via radio signals. In peer communications, all units are identical. There is no centralized control that can, for example, take care of resource or connection management, or provide other support services. In ad-hoc networks, any unit can establish a connection to any other unit in range. Ad-hoc networks are usually based on peer communications. To support the cable replacement scenarios mentioned above, the data traffic over the radio interface must be very flexible. The interface must support both symmetric and asymmetric (in arbitrary direction) traffic flows. In addition, both synchronous traffic like voice as well as asynchronous traffic like web surfing must be supported. In Bluetooth™ this has been realized with a very flexible slot structure without any multi-slot frames or anything akin thereto. The time axis in Bluetooth™ is divided into slots and the units are free to allocate the slots for transmission or reception.
Radio communication systems for personal usage differ from radio systems like the public mobile phone network in that they have to operate in an unlicenced band and have to deal with uncontrolled interference. A suitable band is the ISM (Industrial Scientific and Medical) band at 2.45 GHz, which is globally available. This band provides 83.5 MHz of radio spectrum. Since independent radio connections will share the same spectrum, mutual interference cannot be prevented. In order to obtain 100% data integrity, data communication applies retransmission schemes to retransmit data packets that have been received incorrectly by the recipient.
Numerous automatic retransmission query (ARQ) schemes have been studied in the past, see for example the book “Data Networks” by Bertsekas and Gallagher, by Prentice-Hall, Inc., 1992, ISBN 0-13-201674-5. In principle, there are three types of ARQ schemes of which the others are all derivatives: the stop-and-wait, the go-back-N and the selective-repeat ARQ scheme. In the stop-and-wait scheme the next packet is only transmitted if the previous packet has been acknowledged. In the go-back-N scheme, N packets can be sent before the first packet is checked on correct reception. If not, all N packets are retransmitted irrespective of whether they were correctly received or not. In the selective-repeat ARQ scheme, only the packet that failed is retransmitted and the receiver can request specific packets to be transmitted. For practical reasons, the selective-repeat scheme is usually combined with a go-back-N scheme. The go-back-N and selective-repeat ARQ schemes can be optimized if the round-trip delay over the link is known. In most conventional communication systems, the delay is relatively constant and can be determined.
However, in the flexible radio connections intended to replace cable connections, the delay is not known and can vary considerably. If the radio system can transmit packets of variable length arbitrarily in the forward and backward directions, the delay can vary considerably from packet to packet. Therefore, the delay cannot be a parameter with which the retransmission scheme can be optimized. Moreover, a commercial radio system demands an efficient air protocol that does not pollute the radio spectrum. In particular, retransmission of packets that have already been received correctly should be prevented.
Therefore, there is a need for a data protocol for radio services that permits flexible allocation of traffic flow in both directions, is robust to errors and disturbances on a channel, and can cope with variable delay conditions. There is also a need for a data protocol for radio services that provides a ping-pong protocol suitable for use by two or more units.